Aqueous systems, such as for example coatings containing emulsion polymer binders, typically employ thickeners to obtain the desired degree of viscosity needed for the proper formulation and application of the aqueous system. The general type of thickener used in aqueous systems is referred to in the art by the term "associative." Associative thickeners are so called because the mechanism by which they thicken is believed to involve hydrophobic associations between the hydrophobic moieties on the thickener molecules themselves and/or with other hydrophobic surfaces. A number of different types of associative thickeners are known including, but not limited to hydrophobically-modified polyurethanes, hydrophobically-modified alkali soluble emulsions, hydrophobically-modified hydroxyethyl cellulose or other hydrophobically-modified natural products, and hydrophobically modified polyacrylamides.
Certain of these associative thickeners, such as for example the hydrophobically-modified polyurethane thickeners, are sold as aqueous solutions containing organic cosolvents. The function of the organic cosolvent, such as for example propylene glycol and butyl carbital, is to suppress the viscosity of the aqueous solution containing the associative thickener to allow for ease in its handling before its use as a thickener. While these organic solvents perform their intended function, they possess potential environmental, safety and health disadvantages. Viscosity suppression may also be accomplished by the use of surfactants. While this presents no specific health/environmental hazard, it does degrade paint performance.
U.S. Pat. No. 5,137,571 to Eisenhart et al. discloses a method for reversibly complexing a cyclodextrin, compound with the hydrophobic moieties on a hydrophobically modified thickener to suppress the viscosity of the aqueous solution containing the thickener so that such solutions can be easily handled and then decomplexing the cyclodextrin compound from the thickener to permit the thickener to perform its intended function. Eisenhart et al. disclose that .alpha., .beta., and .gamma. cyclodextrins can be used effectively to suppress the viscosity of the aqueous solutions of hydrophobically modified thickeners. They also disclose that hydroxyethyl- and hydroxypropyl-cyclodextrins are preferred to the unmodified versions of cyclodextrin.
We unexpectedly have found the methyl-.beta.-cyclodextrin, a modified version of cyclodextrin which was not commericially available until about May 1992 gives superior viscosity suppression as compared to the preferred commericially-available hydroxyethyl cyclodextrin and even as compared to the most preferred commericially-available hydroxypropyl cyclodextrin.